System and method for pumping a particle-laden fluid, such as pressurized fracking fluid

ABSTRACT

A system for delivering pressurized fracking fluid to a well includes a high-pressure pump pumping proppant-free fluid, and a double-piston pressure transfer arrangement for transferring pressure from the proppant-free fluid to a fracking fluid. The pressure transfer arrangement includes first and second cylinder assemblies, each having a piston in sliding engagement within a first hollow cylinder so as to define first and second chambers of the first cylinder and third and fourth chambers of the second cylinder. A piston rod interconnects the pistons. A flow selector alternately directs high-pressure proppant-free fluid to the first chamber and the third chamber so as to act on the pistons, thereby applying pressure to fracking fluid within the second and fourth chambers, respectively, for delivery to the well in alternate power strokes. The faces of the pistons facing the first and third chambers have a smaller effective surface area than the faces towards the second and fourth chambers such that pressure in the fracking fluid remains lower than pressure in the proppant-free fluid.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to mining and, in particular, it concernsa system and method for pumping a particle-laden fluid, such aspressurized fracking fluid delivered to a well.

Well completion operations in the oil and gas industry often involvehydraulic fracturing (often referred to as fracking or fracing) toincrease the release of oil and gas in rock formations. Hydraulicfracturing involves pumping a fluid (a “frac fluid” or “fracking fluid”)containing a combination of water, chemicals, and proppant (e.g., sand,ceramics) into a well at high pressures. The high pressures of the fluidincrease crack size and crack propagation through the rock formation,thereby releasing more oil and gas, while the proppant prevents thecracks from closing once the fluid is depressurized. Fracturingoperations use high-pressure pumps to increase the pressure of thefracking fluid. Unfortunately, the proppant in the fracking fluidincreases wear and maintenance on the high-pressure pumps.

US Patent Application Publication No. 2015/0096739 A1 proposes the useof pressure transfer between two fluids to allow the high pressure pumpsto operate on a “clean” fluid without proppant, and then uses thepressure of the clean fluid to drive the proppant-laden fluid. Theproposed systems may however suffer from leakage of proppant-laden fluidinto the clean fluid, leading to wear on the pressure-transfer systemand contamination of the clean fluid.

SUMMARY OF THE INVENTION

The present invention is a system and method for pumping particle-ladenfluid.

According to the teachings of the present invention there is provided, asystem for delivering pressurized fracking fluid to a well, the systemcomprising: (a) a source of proppant-laden fracking fluid; (b) ahigh-pressure pump connected to a source of proppant-free fluid; (c) adouble-piston pressure transfer arrangement, the pressure transferarrangement comprising: (i) a first cylinder assembly comprising a firsthollow cylinder, and a first piston in sliding engagement within thefirst hollow cylinder so as to at least partially define a first chamberand a second chamber; (ii) a second cylinder assembly comprising asecond hollow cylinder, and a second piston in sliding engagement withinthe second hollow cylinder so as to at least partially define a thirdchamber and a fourth chamber; and (iii) at least one piston rodinterconnecting the first piston and the second piston; and (d) a flowselector associated with the high-pressure pump, the first chamber ofthe first cylinder and the third chamber of the second cylinder, theflow selector assuming alternately: (i) a first selector state in whichthe proppant-free fluid from the high-pressure pump is directed to thefirst chamber so as to act on the first piston, thereby applyingpressure to a quantity of the fracking fluid within the second chamberfor delivery to the well in a first power stroke, the first power strokealso expelling a quantity of the proppant-free fluid from the thirdchamber and taking in a quantity of the fracking fluid to the fourthchamber; and (ii) a second selector state in which the proppant-freefluid from the high-pressure pump is directed to the third chamber so asto act on the second piston, thereby applying pressure to a quantity ofthe fracking fluid within the fourth chamber for delivery to the well ina second power stroke, the second power stroke also expelling thequantity of the proppant-free fluid from the first chamber and taking ina quantity of the fracking fluid to the second chamber, wherein thefirst and second pistons have an inner face facing, respectively, thefirst and third chambers and an outer face facing, respectively, thesecond and fourth chamber, the outer face having larger effectivesurface area than the inner face, such that pressure in the frackingfluid remains lower than pressure in the proppant-free fluid.

According to a further feature of an embodiment of the presentinvention, the first cylinder arrangement further comprises a pressurevessel enveloping substantially the entirety of the first hollowcylinder, the pressure vessel defining at least one enveloping volumeselected from the group consisting of: a first enveloping volume influid flow communication so as to form an extension of the firstchamber; and a second enveloping volume in fluid flow communication soas to form an extension of the second chamber, and wherein the secondcylinder arrangement further comprises a pressure vessel envelopingsubstantially the entirety of the second hollow cylinder, the pressurevessel defining at least one enveloping volume selected from the groupconsisting of: a third enveloping volume in fluid flow communication soas to form an extension of the third chamber; and a fourth envelopingvolume in fluid flow communication so as to form an extension of thefourth chamber.

According to a further feature of an embodiment of the presentinvention, there is also provided: (a) a bidirectional hydraulicactuator associated with the flow selector for switching the flowselector between the first selector state and the second selector state;and (b) a hydraulic switch having a switch inlet port for the inflow ofa pressurized hydraulic fluid and having two hydraulic connections tothe hydraulic actuator for actuating the hydraulic actuator, wherein thehydraulic switch is deployed to be actuated by motion of the first andsecond pistons to switch the hydraulic connections of the hydraulicswitch, thereby subsequently actuating the bidirectional hydraulicactuator to switch the flow selector between the first and secondselector states.

According to a further feature of an embodiment of the presentinvention, the hydraulic actuator includes an actuator pistondisplaceable within an actuator cylinder.

According to a further feature of an embodiment of the presentinvention, the switch inlet port is connected to receive proppant-freefluid from the high-pressure pump.

According to a further feature of an embodiment of the presentinvention, the double-piston pressure transfer arrangement is referredto as a master pressure transfer arrangement, and wherein the flowselector is referred to as a master flow selector, the system furthercomprising: (a) a slave double-piston pressure transfer arrangement, thepressure transfer arrangement comprising: (i) a first cylinder assemblycomprising a first hollow cylinder, and a first piston in slidingengagement within the first hollow cylinder so as to at least partiallydefine a first chamber and a second chamber; (ii) a second cylinderassembly comprising a second hollow cylinder, and a second piston insliding engagement within the second hollow cylinder so as to at leastpartially define a third chamber and a fourth chamber; and (iii) atleast one piston rod interconnecting the first piston and the secondpiston; and (b) a slave flow selector associated with the high-pressurepump, the first chamber of the first cylinder and the third chamber ofthe second cylinder, the flow selector assuming alternately: (i) a firstselector state in which the proppant-free fluid from the high-pressurepump is directed to the first chamber so as to act on the first piston,thereby applying pressure to a quantity of the fracking fluid within thesecond chamber for delivery to the well in a first power stroke, thefirst power stroke also expelling a quantity of the proppant-free fluidfrom the third chamber and taking in a quantity of the fracking fluid tothe fourth chamber; and (ii) a second selector state in which theproppant-free fluid from the high-pressure pump is directed to the thirdchamber so as to act on the second piston, thereby applying pressure toa quantity of the fracking fluid within the fourth chamber for deliveryto the well in a second power stroke, the second power stroke alsoexpelling the quantity of the proppant-free fluid from the first chamberand taking in a quantity of the fracking fluid to the second chamber;and (c) a bidirectional slave hydraulic actuator associated with theslave flow selector for switching the slave flow selector between thefirst selector state and the second selector state, wherein the slavehydraulic actuator is associated with the master flow selector or themaster pressure transfer arrangement such that, during the first powerstroke of the master pressure transfer arrangement, proppant-free fluidfrom the high pressure pump is delivered to the slave hydraulic actuatorso as to switch the slave flow selector from the second selector stateto the first selector state and, during the second power stroke of themaster pressure transfer arrangement, proppant-free fluid from the highpressure pump is delivered to the slave hydraulic actuator so as toswitch the slave flow selector from the first selector state to thesecond selector state.

There is also provided according to the teachings of an embodiment ofthe present invention, a method for delivering pressurized frackingfluid to a well, the method comprising the steps of: (a) providing adouble-piston pressure transfer arrangement, the pressure transferarrangement comprising: (i) a first cylinder assembly comprising a firsthollow cylinder, and a first piston in sliding engagement within thefirst hollow cylinder so as to at least partially define a first chamberand a second chamber; (ii) a second cylinder assembly comprising asecond hollow cylinder, and a second piston in sliding engagement withinthe second hollow cylinder so as to at least partially define a thirdchamber and a fourth chamber; and (iii) at least one piston rodinterconnecting the first piston and the second piston; and (b)directing a proppant-free fluid from a high-pressure pump to the firstchamber so as to act on the first piston, thereby applying pressure to aquantity of the fracking fluid within the second chamber for delivery tothe well in a first power stroke, the first power stroke also expellinga quantity of the proppant-free fluid from the third chamber and takingin a quantity of the fracking fluid to the fourth chamber; and (c)directing the proppant-free fluid from the high-pressure pump to thethird chamber so as to act on the second piston, thereby applyingpressure to a quantity of the fracking fluid within the fourth chamberfor delivery to the well in a second power stroke, the second powerstroke also expelling the quantity of the proppant-free fluid from thefirst chamber and taking in a quantity of the fracking fluid to thesecond chamber, wherein the first and second pistons have an inner facefacing, respectively, the first and third chambers and an outer facefacing, respectively, the second and fourth chamber, the outer facehaving larger effective surface area than the inner face, such thatpressure in the fracking fluid remains lower than pressure in theproppant-free fluid.

According to a further feature of an embodiment of the presentinvention, the first cylinder arrangement further comprises a pressurevessel enveloping substantially the entirety of the first hollowcylinder, the pressure vessel defining at least one enveloping volumeselected from the group consisting of: a first enveloping volume influid flow communication so as to form an extension of the firstchamber; and a second enveloping volume in fluid flow communication soas to form an extension of the second chamber, and wherein the secondcylinder arrangement further comprises a pressure vessel envelopingsubstantially the entirety of the second hollow cylinder, the pressurevessel defining at least one enveloping volume selected from the groupconsisting of: a third enveloping volume in fluid flow communication soas to form an extension of the third chamber; and a fourth envelopingvolume in fluid flow communication so as to form an extension of thefourth chamber.

There is also provided according to the teachings of an embodiment ofthe present invention, a system for pumping a particle-laden fluid froma fluid source, the system comprising: (a) a pump connected to a sourceof clean fluid; (b) a double-piston pressure transfer arrangement, thepressure transfer arrangement comprising: (i) a first cylinder assemblycomprising a first hollow cylinder, and a first piston in slidingengagement within the first hollow cylinder so as to at least partiallydefine a first chamber and a second chamber; (ii) a second cylinderassembly comprising a second hollow cylinder, and a second piston insliding engagement within the second hollow cylinder so as to at leastpartially define a third chamber and a fourth chamber; and (iii) atleast one piston rod interconnecting the first piston and the secondpiston; and (c) a flow selector associated with the pump, the firstchamber of the first cylinder and the third chamber of the secondcylinder, the flow selector assuming alternately: (i) a first selectorstate in which the clean fluid from the pump is directed to the firstchamber so as to act on the first piston, thereby applying pressure to aquantity of the particle-laden fluid within the second chamber fordelivery through an outlet in a first power stroke, the first powerstroke also expelling a quantity of the clean fluid from the thirdchamber and taking in a quantity of the particle-laden fluid to thefourth chamber; and (ii) a second selector state in which the cleanfluid from the pump is directed to the third chamber so as to act on thesecond piston, thereby applying pressure to a quantity of theparticle-laden fluid within the fourth chamber for delivery through anoutlet in a second power stroke, the second power stroke also expellingthe quantity of the clean fluid from the first chamber and taking in aquantity of the particle-laden fluid to the second chamber, wherein thefirst and second pistons have an inner face facing, respectively, thefirst and third chambers and an outer face facing, respectively, thesecond and fourth chamber, the outer face having larger effectivesurface area than the inner face, such that pressure in theparticle-laden fluid remains lower than pressure in the clean fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1A is a schematic diagram showing a high-pressure fluid injectionsystem employing a paired cylinder arrangement according to anembodiment of the invention;

FIG. 1B is an enlarged view of a paired cylinder arrangement accordingto an embodiment of the invention;

FIG. 2A is a schematic diagram showing a paired cylinder arrangementincluding a hydraulic switch and flow selector with hydraulic actuatoraccording to an embodiment of the invention;

FIG. 2B is an enlarged view of a paired cylinder arrangement including ahydraulic switch and flow selector with hydraulic actuator according toan embodiment of the invention;

FIG. 3A is a cross-sectional view of a flow selector according to anembodiment of the invention;

FIG. 3B is an enlarged view of a flow selector according to anembodiment of the invention;

FIG. 4 is a cross-sectional view of a hydraulic switch according to anembodiment of the invention;

FIG. 5A is a view of some of the elements of a hydraulic switchaccording to an embodiment of the invention;

FIG. 5B is a cross-sectional view of elements of a hydraulic switchaccording to an embodiment of the invention;

FIGS. 6A-6C show a schematic diagram of the state transitions of a flowselector and a hydraulic switch according to an embodiment of theinvention;

FIG. 7 is a schematic diagram showing a controlled out of phase supplyof pressurized fluid by a plurality of paired cylinder arrangementsaccording to an embodiment of the invention;

FIG. 8 is a schematic diagram showing a master paired cylinderarrangement in connection with a number of successive slave pairedcylinder arrangements for controlling a supply of pressurized fluidaccording to an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a system and method for pumping aparticle-laden fluid, such as fracking fluid delivered at high pressureto a well.

The principles and operation of systems and methods according to thepresent invention may be better understood with reference to thedrawings and the accompanying description.

Referring now to the drawings, FIG. 1A shows a schematic overview of asystem, generally designated 100, constructed and operative according toan embodiment of the present invention applied to the field of fracking,for delivering pressurized fracking fluid to a well so as to fractureand prop-open cracks in rock 102.

Generally speaking, in the exemplary case of fracking, system 100includes a source of proppant-laden fracking fluid 104, a high-pressurepump 106 connected to a source 108 of proppant-free fluid, and adouble-piston pressure transfer arrangement 110. Pressure transferarrangement 110 includes a first cylinder assembly 200-1 having a firsthollow cylinder 204-1, and a first piston 208-1 in sliding engagementwithin first hollow cylinder 204-1 so as to at least partially define afirst chamber 214-1 and a second chamber 216-1. Pressure transferarrangement 110 further includes a second cylinder assembly 200-2 havinga second hollow cylinder 204-2, and a second piston 208-2 in slidingengagement within second hollow cylinder 204-2 so as to at leastpartially define a third chamber 214-2 and a fourth chamber 216-2. Atleast one piston rod 218 interconnects first piston 208-1 and secondpiston 208-2.

A flow selector 300, associated with high-pressure pump 106, firstchamber 214-1 and third chamber 214-2, assumes alternately:

-   -   A first selector state in which the proppant-free fluid from        high-pressure pump 106 is directed to first chamber 214-1 so as        to act on first piston 208-1, thereby applying pressure to a        quantity of the fracking fluid within second chamber 216-1 for        delivery to the well in a first power stroke. This first power        stroke also expels a quantity of the proppant-free fluid from        third chamber 214-1 and takes in a quantity of the fracking        fluid to fourth chamber 216-2.    -   A second selector state in which the proppant-free fluid from        the high-pressure pump 106 is directed to third chamber 214-2 so        as to act on second piston 208-2, thereby applying pressure to a        quantity of the fracking fluid within fourth chamber 216-2 for        delivery to the well in a second power stroke. This second power        stroke also expels the quantity of proppant-free fluid from        first chamber 214-1 and takes in a quantity of the fracking        fluid to second chamber 216-1, ready for a next cycle.

First and second pistons 208-1 and 208-2 have inner faces 210-1 and210-2, respectively, facing first and third chambers 214-1 and 214-2,and outer faces 212-1 and 212-2, respectively, facing second and fourthchambers 216-1 and 216-2. Outer faces 212-1 and 212-2 have largereffective surface areas than inner faces 210-1 and 210-2, such thatpressure in the fracking fluid remains lower than pressure in theproppant-free fluid.

At this stage, it will already be appreciated that, according to certainpreferred embodiments of the present invention, by maintaining thepressure of the proppant-free fluid higher than that of the frackingfluid, any leakage around pistons 208-1 and 208-2 occurs only in theforward direction, from the clean fluid into the fracking fluid, therebyreducing proppant-induce wear or mechanical disruption of the pistons,and avoiding contamination of the proppant-free fluid which is recycledthrough the high pressure pump. This and other advantages of certainembodiments of the present invention will be better understood byreference to the detailed description herein.

The present invention is applicable to any situation in which a fluidcontaining solid particles as a suspension, slurry or admixture of anysort, or with particularly abrasive or corrosive properties, is to bepumped from one location to another, or to be delivered at high pressureto a target location or volume. Examples of relevant fields to which thepresent invention may be applied include, but are not limited to:hydraulic fracturing (“fracking”), pumped hydroelectric energy storage,and geothermal energy recovery. In each case, pressure applied to a“clean fluid” (i.e., a fluid without a mechanically-significant contentof particles) is transferred via the pressure-transfer arrangement ofthe present invention to the particle-laden fluid. The detaileddescription herein is given in the non-limiting exemplary context offracking, where particularly high pressures are required. Theapplication of the invention to other scenarios in whichsolid-particle-laden water or other liquids are to be pumped will beself-explanatory by analogy to the relevant features described herein.

In the example of fracking, the invention provides a solution for highpressure applications. “High pressure” for this purpose is defined asany pressure above about 20 bar, although fracking applicationstypically reach pressures in excess of 100 bar, and often severalhundred bar, up to 1000 bar or more. In fracking processes, highpressure fluid with addition of a proppant is delivered into rock inorder to open up fissures and prop them open. Fracking has applicationsin the field of mining for enhancing the rate at which petroleum (oil)and natural gas can be recovered from subterranean natural reservoirs,and may also be used to improve accessibility to groundwater wells or toprovide access routes for various other industrial or experimentalprocesses. The word “well” is used herein in the description and claimsto refer to any and all scenarios in which a subterranean (or under-sea)channel is used to provide access for extraction of, or access to, asubterranean natural resource. The fracking fluid is typically waterwith an admixture of proppant particles and chemical additives toprovide various desired physical and chemical properties, providingproperties such as increased viscosity, corrosion resistance, resistanceto freezing etc., all as is known in the art. Fluids other than watermay also be used as the basis for the fracking fluid. The proppantparticles may be any suitable granular material, most commonly silicasand or ceramic particles.

Thus, in a typical fracking implementation as illustrated in FIG. 1A,source 104 of fracking fluid includes a low-pressure pump 113 drawingwater from a tank 114 or other water source. A mixer device 116 deliversthe proppant into the fluid flow, as is known in the art, before it isdelivered to the double-piston pressure transfer arrangement 110. Source104 typically provides the low-pressure fracking fluid supply to secondand fourth 216-1 and 216-2 via two respective check valves 118-1 and118-2 that are typically part of a valve block 120 which also includescheck valves 122-1 and 122-2 through which high pressure fracking fluidis delivered during first and second power strokes, respectively, fromchambers 216-1 and 216-2 to the well.

The proppant-free fluid is typically water, but other fluids may also beused. As mentioned above, various preferred implementations of thepresent invention maintain a pressure differential from theproppant-free fluid to the fracking fluid, such that any leakage aroundthe pistons occurs from the clean fluid to the fracking fluid. Thechoice of proppant-free fluid is therefore preferably compatible withthe primary fluid component of the fracking fluid so that any leakage ofthe proppant-free fluid into the fracking fluid does not adverselyaffect the fracking fluid properties.

In order to ensure smooth operation of high pressure pump 106 withoutcavitation at its inlet, a low pressure pump 124 preferably feeds theproppant-free fluid from tank 108 to high pressure pump 106. Fluidexpelled from the first and third chambers 214-1 and 214-2 during theirrespective return strokes is preferably returned via selector 300 totank 108 for recycling as part of a closed-loop system, which may beperiodically or continuously topped up if necessary, or may start withsufficient volume of liquid to accommodate any likely leakage losses.The structure and operation of selector 300 will be detailed furtherbelow.

An implementation of double-piston pressure transfer arrangement 110 isshown in more detail in FIG. 1B. Cylinders 204-1 and 204-2 have aninterior surface defining an interior volume and are open at a singleend 206-1 and 206-2. Hollow cylinders 204-1 and 204-2 are preferablypositioned inside respective pressure vessels 202-1 and 202-2, therebylowering the pressure differential across the walls of cylinders 204-1and 204-2. The envelopment of cylinders 204-1 and 204-2 by respectivepressure vessels 202-1 and 202-2 defines enveloping volumes in therespective spaces between cylinders 204-1 and 204-2 and pressure vessels202-1 and 202-2. It is preferable that substantially the entirety of thesurface area of cylinders 204-1 and 204-2 is enveloped by respectivepressure vessels 202-1 and 202-2. In this configuration, the entiresurface areas of cylinders 204-1 and 204-2 are exposed to high pressurefrom within respective pressure vessels 202-1 and 202-2. This allows forcylinders 204-1 and 204-2 to be made from a lower cost and/or thinnermaterial chosen for its dimensional stability and wear resistance.Particularly preferred materials include stainless steel, compositematerials and various polymer materials. Although the depiction ofpressure vessels 202-1 and 202-2 in FIGS. 1A-1B shows a cylindricalshape with flat ends, other suitable shapes can be used, such as, forexample, a tubular shape with hemispherical ends. Pressure vessels 202-1and 202-2 are made of a material or combination of materials suitablefor withstanding high pressure differentials including, but not limitedto, steel and composite materials.

As mentioned above, each cylinder 204-1 and 204-2 contains areciprocally movable piston 208-1 and 208-2 for moving within therespective internal surfaces of cylinders 204-1 and 204-2. Pistons 208-1and 208-2 have inner faces 210-1 and 210-2 and outer faces 212-1 and212-2. Pistons 208-1 and 208-2 effectively subdivide the internalvolumes of cylinders 204-1 and 204-2 into volumes forming part ofchambers 216-1 and 216-2 for the fracking fluid and volumes forming apart of chambers 214-1 and 214-2 for the proppant-free fluid.Preferably, outer faces 212-1 and 212-2 are of larger effective surfacearea than inner faces 210-1 and 210-2. This difference in the effectivesurface area provides a step-down pressure difference so that a givenpressure of clean fluid introduced into chamber 214-1 or 214-2 resultsin a somewhat lower output pressure of fracking fluid from thecorresponding chamber 216-1 or 216-2. The cylinder arrangement of hollowcylinders 204-1 and 204-2 inside of respective pressure vessels 202-1and 202-2 are hereinafter referred to as cylinder arrangements 200-1 and200-2. It is preferable that cylinder arrangements 200-1 and 200-2 areconnected by a mechanical arrangement, thereby creating a pairedcylinder arrangement, as detailed further below.

Fracking fluid inlet-outlet ports 220-1 and 220-2 are here implementedas connections to pressure vessels 202-1 and 202-2. An additionaladvantage of the double cylinder arrangements 200-1 and 200-2 is thatinlet-outlet ports 220-1 and 220-2 may be placed far from respectiveopen ends 206-1 and 206-2. This allows for the grouping of frackingfluid inlet-outlet ports near the center of arrangement 110 as shown inFIGS. 1A-1B, thereby minimizing the required length of high pressurepipes and avoiding stress caused by the cyclic longitudinal expansionand contraction along the length of pressure vessels 202-1 and 202-2which could otherwise lead to connection reliability issues.Furthermore, the use of double cylinder arrangements ensures that thepressure vessels themselves are not subject to mechanical wear from themoving pistons.

Although the configuration shown here has the fracking fluid chamberextending around the inner cylinder, a similar reduction of the pressuredifferential across the walls of cylinders 204-1 and 204-2 may beachieved by extending the clean fluid chambers 214-1 and 214-2 toenvelope some or all of the exterior of respective cylinders 204-1 and204-2. Where the enveloping volume is provided in part by the frackingfluid chamber and in part by the clean fluid chamber, a relativelynarrow dividing wall (not shown) is provided to seal around the innercylinder and subdivide the enveloping volume.

It is most preferable that the aforementioned mechanical arrangement isimplemented using at least one connecting rod 218 connecting pistons208-1 and 208-2 for simultaneous movement. Rod 218 preferablyinterconnects pistons 208-1 and 208-2 at inner faces 210-1 and 210-2.Rod 218 extends through an aperture or apertures in a central bodyseparating cylinder arrangements 200-1 and 200-2. Due to the pressuredifference between the clean fluid and the fracking fluid, leakage offracking fluid from chambers 216-1 and 216-2 into the clean fluid inchambers 2144 and 214-2 is typically avoided. Nevertheless, pistons208-1 and 208-2 may advantageously include a structure, such as a sealring or the like, which prevents mixing in either direction between theclean fluid in chambers 214-1 and 214-2 and the fracking fluid inchambers 216-1 and 216-2. Pistons 208-1 and 208-2 may be described asmoving between first and second extreme positions. The first position ofpiston 208-1 is preferably adjacent inlet-outlet port 222-1, while thefirst position of piston 208-2 is preferably adjacent inlet-outlet port222-2, i.e., the positions of the pistons nearest the middle of assembly110. The second position of piston 208-1 is preferably adjacent open end206-1 of cylinder 204-1, while the second position of piston 208-2 ispreferably adjacent open end 206-2 of cylinder 204-2. The movement ofpistons 208-1 and 208-2 from their respective first positions to secondpositions is referred to as the pressure stroke of pistons 208-1 and208-2 during which fracking fluid is expelled under pressure fordelivery to the well. The movement of pistons 208-1 and 208-2 from theirrespective second positions to first positions is referred to as thereverse stroke of pistons 208-1 and 208-2. It is apparent that, due tothe mechanical linkage via rod(s) 218, the pressure stroke of piston208-1 coincides with the reverse stroke of piston 208-2, and that thepressure stroke of piston 208-2 coincides with the reverse stroke ofpiston 208-1.

Although the use of one or more common piston rod rigidly linkingpistons 208-1 and 208-2 is a particularly preferred implementation usedthroughout the description, it should be noted that other forms ofmechanical linkage between the pistons to ensure simultaneous andopposite motion may also be used. Such options may be of particularvalue where a compact side-by-side deployment of the pair of cylindersis preferred to the coaxial arrangement illustrated herein. Suchmechanical linkages can readily be implemented using a lever armsconfiguration with pivotally mounted drive rods linked to the pistons.

As further seen in FIG. 1A, proppant-free fluid inlet-outlet ports 222-1and 222-2 are provided in chambers 214-1 and 214-2. Proppant-free fluidinlet-outlet ports 222-1 and 222-2 are in fluid flow connection withhigh-pressure pump 106 via a flow selector 300. Flow selector 300 may bea valve arrangement connected to an actuator 350. Actuator 350 may beconnected to a controller 112 which is associated with sensors(illustrated schematically as a sensor 116) for sensing the positions ofpiston 208-1 and/or piston 208-2. The sensors may include transducers orthe like located adjacent to the first and second positions of piston208-1 and/or piston 208-2, a laser range finder for sensing proximity ofone or both pistons to the end of the cylinders, or various other linearencoder or position sensor arrangements for sensing the position of thepistons or piston rods. Sensing the positions of pistons 208-1 and 208-2is preferably used for timing the stroke reversal of cylinderarrangements 200-1 and 200-2, i.e. the change from pressure stroke toreverse stroke of cylinder arrangement 200-1 and the simultaneous changefrom reverse stroke to pressure stroke of cylinder arrangement 200-2,and vice versa. Flow selector 300 controls the inflow and outflow ofproppant-free fluid to and from inlet-outlet ports 222-1 and 222-2 asactuated by actuator 350. Fracking fluid is correspondingly taken up andexpelled under pressure from chambers 216-1 and 216-2 through the checkvalves of valve block 120. Although described thus far in animplementation using an arrangement of check valves for controlling theinflow and outflow of fracking fluid to and from chambers 216-1 and216-2, other embodiments are possible in which a switchable valvearrangement (not shown) controls the inflow and outflow of the frackingfluid. In such embodiments, the valve may be actuated by actuator 350 toensure synchronization between the inflow/outflow of fracking fluid withthe inflow/outflow of the proppant-free fluid.

The operation of the system as shown in FIGS. 1A-1B will now bedescribed. Proppant-free fluid from tank 108, e.g., clean water, issupplied via feed pump 124 to high pressure pump 106. Flow selector 300directs this flow alternately to one of chambers 214-1 and 214-2 whilelow pressure clean water is drained from the other of these chambers viaflow selector 300 back to tank 108. According to the flow pathsillustrated by solid arrows in FIG. 1A, high pressure clean water iscurrently being delivered to chamber 214-1, thereby driving piston 208-1to the left as shown and delivering fracking fluid from chamber 216-1 athigh pressure via outlet 220-1 and check valve 122-1 to be delivered tothe target volume (rocks 102 or the well). The power-stroke motion ofpiston 208-1 is also conveyed via piston rod(s) 218 to displace piston208-2 to the left in its return stroke, thereby simultaneously expellingfrom chamber 214-2 the clean water introduced during the previous powerstroke via selector 300 and drawing in fracking fluid via check valve118-2, to fill chamber 216-2.

When piston 208-1 nears its second position, for example as identifiedby position sensors detecting the position of piston 208-1 and/or piston208-2, controller 112 preferably sends a control signal to actuator 350to actuate flow selector 300 to change the inflow and outflow of cleanwater to and from inlet-outlet ports 222-1 and 222-2. In such anarrangement, flow selector 300 is preferably actuated prior to piston208-1 reaching the limit of its range of motion, thereby minimizing anymomentary dead time which may occur during the direction reversal, andany consequent fluctuation in the output flow rate from the cylinderarrangement. After actuation of flow selector 300, fluid connections toand from cylinder arrangements 200-1 and 200-2 are reversed wherebyhigh-pressure fresh water from pump 106 is supplied to chamber 214-2 todrive piston 208-2 from its first position towards its second position.Fracking fluid from chamber 216-2 is pumped through inlet-outlet port220-2 and check valve 122-2 for delivery to the target location in thewell. New fracking fluid is simultaneously drawn in via check valve118-1 and inlet-outlet port 220-1 from fracking fluid source 104 tochamber 216-1, and clean water is discharged from chamber 214-1 throughinlet-outlet port 222-1. When piston 208-2 nears its second position andpiston 208-1 nears its first position, the position of piston 208-1and/or piston 208-2 are sensed via the position sensors or the like andcontroller 112 sends a control signal to actuator 350 to actuate flowselector 300 to change the inflow and outflow of clean water to and frominlet-outlet ports 222-1 and 222-2 as shown in the illustrated firstposition and the pressure transfer cycle repeats.

Parenthetically, although illustrated herein in one particularlypreferred implementation (described further below) as a tube-likeselector, flow selector 300 may be implemented in any of a number ofconfigurations. By way of one additional non-limiting example, a rotaryselector may be used in which arcuate channels alternately connect thehigh pressure clean water connection and the clean water returnconnection with each of the pressurized drive-stroke chambers of thecylinder arrangements.

In certain preferred implementations, it may be preferable to implementthe sensing and actuation process for reversing the direction of pistons208-1 and 208-2 using a reduced number of electro-mechanical components.In certain such cases, it is preferred that flow selector 300 andactuator 350 are operated by a hydraulic switching arrangement withoutrequiring additional sensors or electronic actuators.

According to certain preferred embodiments, flow selector 300 ishydraulically actuated and includes a hollow flow selector cylinderassembly 301 which extends between first and second ends 302-1 and302-2, and which is typically assembled from a number of differentcylindrical and branched sections, as discussed below. With reference toFIGS. 2A-3B, selector 300 as illustrated here has a high-pressure cleanwater inlet port 304 in fluid flow connection with pump 106, and firstand second clean water drainage outlet ports 306-1 and 306-2 in fluidflow connection with a discharge conduit returning the water to tank 108(FIG. 1a ). First and second clean water inlet-outlet ports 307-1 and307-2 are in fluid flow connection with clean water inlet-outlet ports222-1 and 222-2 respectively.

According to certain particularly preferred implementations, flowselector 300 is integrated with a hydraulic actuator 350. In this case,at one end of the selector, an integrated hydraulic actuator arrangement350 includes a piston 308 displaceable within an actuator cylinderportion 309 so as to displace a selector rod 316 (which may be assembledfrom a number of separable sections, as shown) between first and secondpositions within cylinder assembly 301. Piston 308 has an inner face 310and an outer face 312. It is preferred that piston 308 includes astructure, such as a seal ring or the like, sealing against the wall ofcylinder portion 309. Cylinder portion 309 is delineated by first andsecond non-moveable seals 305-1 and 305-2. The volume between secondnon-moveable seal 305-2 and outer face 312 is referred to as firstvolume 303-1. The volume between inner face 310 and first non-moveableseal 305-1 is referred to as second volume 303-2. Piston 308 has a firstposition preferably adjacent to second non-moveable seal 305-2. Piston308 has a second position preferably adjacent to first non-moveable seal305-1.

Optionally, selector 300 may include an extension portion 318 withinwhich moves an extension rod 320, interconnected so as to move togetherwith selector rod 316. Provision of extension rod 320 equalizes theeffective surface area on both sides of the piston 308, and provides aconvenient location for a sensor for sensing the current state ofselector 300 as an input to the control system. It should be noted,however, the extension rod 320 is not essential, and an asymmetrybetween the surface areas on the two sides of piston 308 is generallynot considered problematic.

In the preferred implementation illustrated here, cylinder assembly 301contains first and second reciprocally movable seals 314-1 and 314-2 formoving within cylinder assembly 301. Movable seals 314-1 and 314-2 mayhave a piston-like structure, although axially-closing seals arepreferred to radially sliding seals, as detailed below with reference toFIG. 3b . Seal 314-1 has a first position preferably interposed betweenoutlet port 306-1 and inlet-outlet port 307-1. Seal 3144 has a secondposition preferably interposed between inlet-outlet port 307-1 and inletport 304. Seal 314-2 has a first position preferably interposed betweenoutlet port 306-2 and inlet-outlet port 307-2. Seal 314-2 has a secondposition preferably interposed between inlet-outlet port 307-2 and inletport 304. Piston 308 and seals 314-1 and 314-2 are interconnected forsimultaneous movement. Selector rod 316 interconnects piston 308 andseals 314-1 and 314-2. Selector rod 316 extends through apertures innon-moveable seal arrangements 305-1 and 305-2. Preferably, a seal ringor the like at the apertures of non-moveable seals 305-1 and 305-2provides a sealing engagement between rod 316 and non-moveable seals305-1 and 305-2 while allowing for axial movement of rod 316. Asmentioned above, selector rod 316 may be formed from multiple sections,with each section interconnecting different components of flow selector300. For example, a first section of rod 316 may be used to connect toouter face 312 of piston 308. A second section of rod 316 may then beused to interconnect inner face 310 of piston 308 and seal 314-1. Athird section of rod 316 may then be used to interconnect seals 314-1and 314-2. The movement of piston 308 displacing rod 316 acts as ahydraulic actuator. The simultaneous movement of pistons 308 and seals314-1 and 314-2 effects the transitioning of flow selector 300 betweenfirst and second states. When seal 314-1 is in its first position andseal 314-2 is in its second position as shown in FIGS. 2A, 2B and 3A,flow selector 300 is in a first state. Likewise, when seal 314-1 is inits second position and seal 314-2 is in its first position, flowselector 300 is in its second state. When flow selector 300 is in thefirst state, clean water from pump 106 is supplied to chamber 214-1through inlet port 304, inlet-outlet port 307-1 and inlet-outlet port222-1. Simultaneously, low pressure clean water is discharged fromchamber 214-2 through inlet-outlet port 222-2, inlet-outlet port 307-2,outlet port 306-2, and is discharged to tank 108. When flow selector 300is in the second state, clean water from pump 106 is supplied to chamber214-2 through inlet port 304, inlet-outlet port 307-2 and inlet-outletport 222-2. Simultaneously, low pressure clean water is discharged fromchamber 214-1 through inlet-outlet port 222-1, inlet-outlet port 307-1,outlet port 306-1, and is discharged to tank 108.

According to a particularly preferred implementation illustrated here,corresponding to a further aspect of the present invention applicationto flow selectors, the sealing surfaces of sliding seals 314-1 and 314-2are provided on axially facing closure surfaces of the sliding sealsrather than by using radial seals bearing on the inside of a cylinder.Specifically, hollow cylinder assembly 301 is shown here with sectionshaving two differing internal diameters, namely sections 324 having afirst internal diameter and sections 326 having a second internaldiameter, where the first internal diameter is larger than the secondinternal diameter. The locations within hollow cylinder assembly 301 inwhich changes of internal diameter occur form a plurality of steps 328defining the first and second positions of seals 314-1 and 314-2. Asshown in FIGS. 3A-3B, seals 314-1 and 314-2 are movable within thesections 324 of cylinder assembly 301 having the first diameter and haveannular sealing rings 315 on the axially-facing end surfaces of theseals configured for sealing against steps 328, thereby defining theflow paths of the proppant-free fluid through flow selector 300 aspreviously described. By avoiding use of outwardly-pressing sealsagainst the inner surface of cylinder assembly 301, problems ofreliability due to wear of seals 314-1 and 314-2 passing over lateralopenings 220-1 and 220-2 are greatly reduced.

According to a variant implementations illustrated with reference toFIGS. 2A-2B, 4, 5A-5B and 6A-6C, as an alternative or supplement tosensor(s) 116 and controller 112, in certain preferred embodiments, thehydraulic actuator 350 of flow selector 300 is actuated by a hydraulicswitch 400. According to certain particularly preferred implementationsillustrated here, hydraulic switch 400 is mechanically integrated withcylinder arrangement 110 so as to be actuated towards the end of eachstroke of piston 208-1 or 208-2. It should be noted that the use of ahydraulic switch 400 to trigger operation of an actuator 350 whichcauses a change in state of flow selector 300 offers significantadvantages over direct actuation of a flow selector by motion of thepistons. Most notably, since no change in state of selector 300 occursuntil after the state of hydraulic switch 400 has been completelyreversed, potential issues of “stalling” at some intermediate positionduring the state reversal are avoided.

Referring now to FIGS. 2A-2B, 4, 5A-5B and 6A-6C, one particularlypreferred implementation of hydraulic switch 400 as illustrated hereincludes a hollow hydraulic switch chamber 402 with a chamber wall 404having first and second ends. Preferably, chamber wall 404 includes afirst at least one aperture 406-1, a second at least one aperture 406-2,a third at least one aperture 406-3, a fourth at least one aperture406-4, and a fifth at least one aperture 406-5. Apertures 406-1, 406-2,406-3, 406-4 and 406-5 are preferably arranged annularly around chamberwall 404 as depicted in FIGS. 4, 5A and 5B. Chamber wall 404 preferablyincludes a first switch non-moveable seal 407-1, a second switchnon-moveable seal 407-2, a third switch non-moveable seal 407-3, afourth switch non-moveable seal 407-4, a fifth switch non-moveable seal407-5, and sixth switch non-moveable seal 407-6. The preferredinterposed arrangement of the apertures and non-movable seals is asshown in FIGS. 5A-5B. The first end of chamber wall 404 is adjacent seal407-1, and second end of chamber wall 404 is adjacent seal 407-6.

A hydraulic fluid inlet port 408 is provided adjacent to aperture 406-1.Pressurized hydraulic fluid, which may be an oil or may be high or lowpressurized clean water taken from some suitable point in the system, issupplied to inlet port 408 from a hydraulic fluid source. The hydraulicfluid is preferably pressurized to at least 5 bar. Where a dedicatedhydraulic fluid is used, the fluid is pressurized by a suitable device,such as a pressure supply pump 410 or the like. First and secondactuator control inlet-outlet ports 412-1 and 412-2 are providedadjacent to apertures 406-3 and 406-2 respectively. Inlet-outlet port412-1 is in fluid flow connection with outer face 312 of flow selectorpiston 308. Inlet-outlet port 412-2 is in fluid flow connection withinner face 310 of flow selector piston 308. First and second hydraulicfluid drain outlet ports 414-1 and 414-2 are provided adjacent apertures406-4 and 406-5 respectively. Outlet ports 414-1 and 414-2 arepreferably in fluid flow connection with a hydraulic fluid dischargeconduit 416 connected to a hydraulic fluid dump which, in the case of anoil based hydraulic system, is typically the reservoir supplying pump410. Chamber 402 contains a reciprocally movable switching seal 418 formoving within chamber wall 404. It is preferred that seal 418 has apiston like structure. Switching seal 418 has a first positionpreferably adjacent inlet-outlet port 412-1 and a second positionpreferably adjacent inlet-outlet port 412-2. Seal 418 is shown hereintegrated with a switching rod 420 for movement within chamber wall404. As shown in FIG. 5B, rod 420 is also integrated with narrow 426,428 and wide 422, 424 diameter sections. Wide diameter sections 422 and424 act as sliding seals within chamber 402. When seal 418 is in itsfirst position, seal 422 prevents the flow of hydraulic fluid fromaperture 406-1 to aperture 406-5, thus preventing the flow from inletport 408 to outlet port 414-2. When seal 418 is in its second position,seal 424 prevents the flow of hydraulic fluid from aperture 406-1 toaperture 406-4, thus preventing the flow from inlet port 408 to outletport 414-1. Chamber 402 is preferably interposed between chambers 214-1and 214-2. Seal 422 preferably extends out of the first end of chamber402 through an aperture when seal 418 is in the second position.Likewise, seal 424 extends through the second end of chamber 402 whenseal 418 is in the first position. Thus, rod 420 is moved by contactwith pistons 208-1 and 208-2 towards the end of their range of motion.The relative positioning of apertures and switch seals dictates thepossible flow paths the hydraulic fluid can traverse through inlet port408, inlet-outlet ports 412-1 and 412-2, and outlet ports 414-1 and414-2. The movement of switching seal 418 achieves switching ofhydraulic switch 400 between first and second states.

When hydraulic switch 400 is in the first state, pressurized hydraulicfluid from a hydraulic fluid source is supplied to volume 303-2 ofcylinder assembly 301 through inlet-outlet port 412-1, aperture 406-3,aperture 406-1, and inlet port 408. Simultaneously, hydraulic fluid isdischarged from volume 303-1 through inlet-outlet port 412-2, aperture406-2, aperture 406-5, outlet port 414-2 and hydraulic fluid dischargeconduit 416 connected to a hydraulic fluid dump. When hydraulic switch400 is in the second state, pressurized hydraulic fluid from a hydraulicfluid source is supplied to volume 303-1 of cylinder assembly 301through inlet-outlet port 412-2, aperture 406-2, aperture 406-1, andinlet port 408. Simultaneously, hydraulic fluid is discharged fromvolume 303-2 through inlet-outlet port 412-1, aperture 406-3, aperture406-4, outlet port 414-1 and hydraulic fluid discharge conduit 416connected to a hydraulic fluid dump.

As noted, hydraulic switch is interposed between pistons 208-1 and 208-2such that pistons 208-1 and 208-2 move switching rod 420 back and forth.The arrangement preferably operates as a bistable “flip-flop” where, asseal 418 passes the central position of switch 400 in either direction,pressure from the pressure source carries the seal 418 and itsassociated switching rod 420 quickly to the end of its motion, therebytriggering the change of state of actuator 350 and hence of selector300. This arrangement is effective to avoid the risk of stalling of thecylinder arrangement in any “dead” intermediate state during switchingfrom pressure stroke to reverse stroke and vice versa.

Although the switching arrangement described thus far has pertained to ahydraulic switch using a hydraulic fluid such as oil or the like fordriving the hydraulic switch, other embodiments are possible in whichthe hydraulic fluid used to drive hydraulic switch 400 is pressurizedwater. The pressurized water may be taken from the output of lowpressure pump 124, or from high pressure pump 106.

Operation of the cylinder arrangement of FIGS. 2A-2B is essentiallysimilar to that of the system of FIGS. 1A-1B as described above, otherthan in the autonomous mechano-hydraulic reversal functionality providedby hydraulic switch 400 operating actuator 350 to change the state ofselector 300. This functionality is presented schematically withreference to FIGS. 6A-6C. Specifically, as shown in FIG. 6A, as piston208-1 moves to the left and nears the end of its stroke, piston 208-2contacts switching rod 420 and begins to displace the switching rod andassociated switching seals 418, 422 and 424 to the left. As the centralseal passes fluid inlet 408, it is urged by fluid pressure to move tothe fully-displaced left hand position as shown in FIG. 6B, therebyconnecting pressure inlet 408 to port 412-1, and thereby to volume303-2. FIG. 6B represents the state immediately after the state ofswitch 400 has flipped, but before the resulting flow has changed thestate of the selector. FIG. 6C shows the state soon thereafter, afterthe actuator of selector 300 has caused the selector to switch states,thereby beginning the reverse (left-to-right as shown) power stroke.

As noted, outer faces 212-1 and 212-2 are preferably of effectivesurface area larger than inner faces 210-1 and 210-2, thereby achievinga corresponding slight pressure reduction to prevent leakage of frackingfluid into the proppant-free fluid circuit. Preferably, outer faces212-1 and 212-2 are 1%-5% larger than inner faces 210-1 and 210-2. Thedifference in effective surface area typically corresponds to thecross-sectional area of piston rod(s) used to interconnect pistons 208-1and 208-2, and the number and size of the piston rods is chosenaccordingly.

Working pressures for the power strokes of the pressure transfer systemin fracking applications are typically in the order of several hundredbar, and may approach or exceed 1000 bar. However, the pressuredifferential between the clean fluid in chambers 214-1 and 214-2 and thefracking fluid in chambers 216-1 and 216-2 is relatively small,preferably less than 10% of the working output pressure, and typicallyroughly 5% of the working pressure. The “double-walled” structure ofcylinder arrangements 200-1 and 200-2 with substantially the entirety ofthe cylinders surrounded by pressure vessel chambers allows forcylinders 204-1 and 204-2 to be manufactured from lower strengthmaterials than could be used for cylinders which need to withstandlarger pressure differentials. This greatly reduces manufacturing costsby separating the requirements of the precision piston-cylinder geometryfrom the load-bearing requirements applicable to pressure vessels 202-1and 202-2. The pressure vessels themselves do not require precisionsurfaces for sliding contact with a piston, and can therefore beproduced by lower precision manufacturing techniques and without highquality surface finishing. As previously noted, the double-walledstructure of cylinder arrangements 200-1 and 200-2 also allows for ports220-1 and 220-2 to be located farther from open ends 206-1 and 206-2,for example, in the middle third of the total length of the combineddual cylinder assembly. This greatly reduces the risks of connectionfailure due to cyclic expansion and contraction along the length of thecylinder as it experiences cyclic pressure changes.

In cases where a plurality of paired cylinder arrangements HO areoperating in parallel, it may be preferable to ensure that the pistonsmove out-of-phase with each other, i.e., changing directions atdifferent times, in order to minimize the overall effect of any flowfluctuations which may occur during reversal of the piston direction atthe end of each stroke. In the case of a system with electronic controlof the flow selectors, out-of-phase operation is achieved simply bystaggering the control signals for reversal of the different cylinders.As previously described, it is preferred that the positioning of atleast one of the pistons of each paired cylinder arrangement is detectedby a sensor arrangement. Referring to FIG. 7, as an example, anarrangement of two cylinder arrangement pairs, each with first andsecond cylinder arrangements 200-1 and 200-2 is considered. In thisexample, pistons 208-1A and 208-1B are mounted with linear encoders 116Aand 116B, respectively, shown here only schematically. In this example,when in phase motion of pistons 208-1A and 208-1B is detected,controller 112 sends a control signal to one of the actuators, forexample, actuator 350A, to actuate the corresponding flow selector 300Aslightly earlier than would otherwise be required in order to ensurenon-simultaneous switching of direction of the two cylinder assemblies.

When employing autonomous hydraulically-actuated stroke reversal, forexample according to the system of FIGS. 2A-6C, if it is consideredpreferable to take precautions against simultaneous reversal of thepiston direction for two cylinder assemblies, an alternative approachfor ensuring out-of-phase operation of a plurality of energy recoverysubsystems obviates the need for electronic control by employing acascade approach to the hydraulic control where reversal of the strokedirection in a master cylinder assembly triggers sequential reversal ofstroke direction in a chain of slave subsystems. This facilitates animplementation with all-hydraulic control. By way of non-limitingexample, a chain configuration with three cylinder arrangements is shownin FIG. 8. In such a configuration, a master cylinder arrangement actsto control the stroke reversal of subsequent paired cylinderarrangements. This control cylinder arrangement is referred to herein asthe master cylinder arrangement 200A. Master cylinder arrangement 200Apreferably operates according to the description of paired cylinderarrangement 110, with stroke direction reversal either by a combinationof a flow selector 300 and a hydraulic switch 400 arrangement aspreviously described or by any other hydraulic or electronic controlarrangement suitable for a single paired cylinder arrangement. Eachsubsequent cylinder arrangement in the chain configuration is referredto herein as a slave cylinder arrangement. Each slave cylinderarrangement is structurally and operationally similarly to mastercylinder arrangement 200A, with the exception that slave cylinderarrangements typically do not include a hydraulic switch 400 foractuating flow selectors 300B and 300C. Instead, the state switching offlow selector 300 for each slave subsystem is performed by pressurederived from the power stroke of the preceding cylinder arrangement inthe chain which is delivered to the actuator of the selector for theslave cylinder arrangement. With reference to FIG. 8, the hydraulicactuator chambers of the flow selector 300B of the first slave cylinderarrangement 200B are in fluid flow communication, respectively, withproppant-free fluid chambers 214-1A and 214-2A of master cylinderarrangement 200A. In this configuration, master cylinder arrangement200A actuates flow selector 300B to change the inflow and outflow ofproppant-free fluid.

As previously described, the pressure stroke of piston 208-1A coincideswith the simultaneous reverse stroke of piston 208-2A. A fluidconnection is here provided from chamber 214-1A to volume 303-2B inorder to actuate selector 300B to transition from its second state toits first state. This in turn directs high-pressure clean water frompump 106 to chamber 214-1B, thereby moving piston 208-1B in its powerstroke from its first position to its second position. When assembly200A reverses direction, the pressure stroke of piston 208-2A beginswhen high-pressure proppant-free fluid is delivered to chamber 214-2A,which is also in fluid connection with volume 303-1B in order to actuateselector 300B to transition from its first state to its second state.This in turn switches the stroke direction of assembly 200B, directingproppant-free fluid from pump 106 to chamber 2142B, and thereby movingpiston 208-2B in a power stroke from its first position to its secondposition. The direction switching of pistons 208-1B and 208-2B thuscomes at a delay relative to the direction switching of pistons 208-1Aand 208-2A corresponding to the time taken for the pressure of the powerstroke of assembly 200A to operate the actuator to change the state ofthe selector of assembly 200B. A similar interconnection is providedbetween assembly 200B and 200C, such that direction reversal of assembly200C is triggered by, and occurs just after, direction reversal ofassembly 200B. This cascade control scheme can be extended to a largenumber of cylinder assemblies, and is preferable deployed using 2-6cylinder assemblies, and most preferably four cylinder assemblies.

The delays between direction switching of successive cylinderarrangements can be adjusted by modifying the flow impedance of theconduits interconnecting flow selectors, but can reasonably be kept asshort as possible. Although not depicted in FIG. 8, it should also beapparent that each proppant-free fluid inlet port 304A, 304B, and 304Cis in fluid flow connection with high-pressure pump 106, or a number ofsuch pumps operating in parallel.

As previously mentioned, the chain configuration shown in FIG. 8 isadvantageous in that it does not require electronic equipment formaintaining average flow with low fluctuation. Furthermore, theproppant-free fluid such as water is used as the motive fluid foroperating all slave cylinder arrangements, thereby reducing the need forpressure pumps for hydraulic fluid. As mentioned above, the number ofcylinder arrangements is not limited to the number of arrangementdepicted in FIG. 8.

It should be noted that the various aspects of the present inventiondescribed herein may each be used to advantage independently of otheraspects of the invention as presented herein. For example, the varioushydraulic control solutions presented herein, although presented in aparticularly preferred context of the double-walled cylinder structuresof the present invention, may also be used to advantage with otherwiseconventional single-walled cylinder constructions.

It will be appreciated that the above descriptions are intended only toserve as examples, and that many other embodiments are possible withinthe scope of the present invention as defined in the appended claims.

What is claimed is:
 1. A system for delivering pressurized frackingfluid to a well, the system comprising: (a) a source of proppant-ladenfracking fluid; (b) a high-pressure pump connected to a source ofproppant-free fluid; (c) a double-piston pressure transfer arrangement,said pressure transfer arrangement comprising: (i) a first cylinderassembly comprising a first hollow cylinder, and a first piston insliding engagement within said first hollow cylinder so as to at leastpartially define a first chamber and a second chamber; (ii) a secondcylinder assembly comprising a second hollow cylinder, and a secondpiston in sliding engagement within said second hollow cylinder so as toat least partially define a third chamber and a fourth chamber; and(iii) at least one piston rod interconnecting said first piston and saidsecond piston; and (d) a flow selector associated with saidhigh-pressure pump, said first chamber of said first cylinder and saidthird chamber of said second cylinder, said flow selector assumingalternately: (i) a first selector state in which the proppant-free fluidfrom said high-pressure pump is directed to said first chamber so as toact on said first piston, thereby applying pressure to a quantity of thefracking fluid within said second chamber for delivery to the well in afirst power stroke, said first power stroke also expelling a quantity ofthe proppant-free fluid from said third chamber and taking in a quantityof the fracking fluid to said fourth chamber; and (ii) a second selectorstate in which the proppant-free fluid from said high-pressure pump isdirected to said third chamber so as to act on said second piston,thereby applying pressure to a quantity of the fracking fluid withinsaid fourth chamber for delivery to the well in a second power stroke,said second power stroke also expelling the quantity of theproppant-free fluid from said first chamber and taking in a quantity ofthe fracking fluid to said second chamber, wherein said first and secondpistons have an inner face facing, respectively, said first and thirdchambers and an outer face facing, respectively, said second and fourthchamber, said outer face having larger effective surface area than saidinner face, such that pressure in said fracking fluid remains lower thanpressure in said proppant-free fluid.
 2. The system of claim 1, whereinsaid first cylinder arrangement further comprises a pressure vesselenveloping substantially the entirety of said first hollow cylinder,said pressure vessel defining at least one enveloping volume selectedfrom the group consisting of: a first enveloping volume in fluid flowcommunication so as to form an extension of said first chamber; and asecond enveloping volume in fluid flow communication so as to form anextension of said second chamber, and wherein said second cylinderarrangement further comprises a pressure vessel enveloping substantiallythe entirety of said second hollow cylinder, said pressure vesseldefining at least one enveloping volume selected from the groupconsisting of: a third enveloping volume in fluid flow communication soas to form an extension of said third chamber; and a fourth envelopingvolume in fluid flow communication so as to form an extension of saidfourth chamber.
 3. The system of claim 1, further comprising: (a) abidirectional hydraulic actuator associated with said flow selector forswitching said flow selector between said first selector state and saidsecond selector state; and (b) a hydraulic switch having a switch inletport for the inflow of a pressurized hydraulic fluid and having twohydraulic connections to said hydraulic actuator for actuating saidhydraulic actuator, wherein said hydraulic switch is deployed to beactuated by motion of said first and second pistons to switch saidhydraulic connections of said hydraulic switch, thereby subsequentlyactuating said bidirectional hydraulic actuator to switch said flowselector between said first and second selector states.
 4. The system ofclaim 3, wherein said hydraulic actuator includes an actuator pistondisplaceable within an actuator cylinder.
 5. The system of claim 3,wherein said switch inlet port is connected to receive proppant-freefluid from said high-pressure pump.
 6. The system of claim 1, whereinsaid double-piston pressure transfer arrangement is referred to as amaster pressure transfer arrangement, and wherein said flow selector isreferred to as a master flow selector, the system further comprising:(a) a slave double-piston pressure transfer arrangement, said pressuretransfer arrangement comprising: (i) a first cylinder assemblycomprising a first hollow cylinder, and a first piston in slidingengagement within said first hollow cylinder so as to at least partiallydefine first chamber and a second chamber; (ii) a second cylinderassembly comprising a second hollow cylinder, and a second piston insliding engagement within said second hollow cylinder so as to at leastpartially define a third chamber and a fourth chamber; and (iii) atleast one piston rod interconnecting said first piston and said secondpiston; and (b) a slave flow selector associated with said high-pressurepump, said first chamber of said first cylinder and said third chamberof said second cylinder, said flow selector assuming alternately: (i) afirst selector state in which the proppant-free fluid from saidhigh-pressure pump is directed to said first chamber so as to act onsaid first piston, thereby applying pressure to a quantity of thefracking fluid within said second chamber for delivery to the well in afirst power stroke, said first power stroke also expelling a quantity ofthe proppant-free fluid from said third chamber and taking in a quantityof the fracking fluid to said fourth chamber; and (ii) a second selectorstate in which the proppant-free fluid from said high-pressure pump isdirected to said third chamber so as to act on said second piston,thereby applying pressure to a quantity of the fracking fluid withinsaid fourth chamber for delivery to the well in a second power stroke,said second power stroke also expelling the quantity of theproppant-free fluid from said first chamber and taking in a quantity ofthe fracking fluid to said second chamber; and (c) a bidirectional slavehydraulic actuator associated with said slave flow selector forswitching said slave flow selector between said first selector state andsaid second selector state, wherein said slave hydraulic actuator isassociated with said master flow selector or said master pressuretransfer arrangement such that, during said first power stroke of saidmaster pressure transfer arrangement, proppant-free fluid from said highpressure pump is delivered to said slave hydraulic actuator so as toswitch said slave flow selector from said second selector state to saidfirst selector state and, during said second power stroke of said masterpressure transfer arrangement, proppant-free fluid from said highpressure pump is delivered to said slave hydraulic actuator so as toswitch said slave flow selector from said first selector state to saidsecond selector state.
 7. A method for delivering pressurized frackingfluid to a well, the method comprising the steps of: (a) providing adouble-piston pressure transfer arrangement, said pressure transferarrangement comprising: (i) a first cylinder assembly comprising a firsthollow cylinder, and a first piston in sliding engagement within saidfirst hollow cylinder so as to at least partially define a first chamberand a second chamber; (ii) a second cylinder assembly comprising asecond hollow cylinder, and a second piston in sliding engagement withinsaid second hollow cylinder so as to at least partially define a thirdchamber and a fourth chamber; and (iii) at least one piston rodinterconnecting said first piston and said second piston; and (b)directing a proppant-free fluid from a high-pressure pump to said firstchamber so as to act on said first piston, thereby applying pressure toa quantity of the fracking fluid within said second chamber for deliveryto the well in a first power stroke, said first power stroke alsoexpelling a quantity of the proppant-free fluid from said third chamberand taking in a quantity of the fracking fluid to said fourth chamber;and (c) directing the proppant-free fluid from the high-pressure pump tosaid third chamber so as to act on said second piston, thereby applyingpressure to a quantity of the fracking fluid within said fourth chamberfor delivery to the well in a second power stroke, said second powerstroke also expelling the quantity of the proppant-free fluid from saidfirst chamber and taking in a quantity of the fracking fluid to saidsecond chamber, wherein said first and second pistons have an inner facefacing, respectively, said first and third chambers and an outer facefacing, respectively, said second and fourth chamber, said outer facehaving larger effective surface area than said inner face, such thatpressure in said fracking fluid remains lower than pressure in saidproppant-free fluid.
 8. The method of claim 7, wherein said firstcylinder arrangement further comprises a pressure vessel envelopingsubstantially the entirety of said first hollow cylinder, said pressurevessel defusing at least one enveloping volume selected from the groupconsisting of: a first enveloping volume in fluid flow communication soas to form an extension of said first chamber; and a second envelopingvolume in fluid flow communication so as to form an extension of saidsecond chamber, and wherein said second cylinder arrangement furthercomprises a pressure vessel enveloping substantially the entirety ofsaid second hollow cylinder, said pressure vessel defining at least oneenveloping volume selected from the group consisting of: a thirdenveloping volume in fluid flow communication so as to form an extensionof said third chamber; and a fourth enveloping volume in fluid flowcommunication so as to form an extension of said fourth chamber.
 9. Asystem for pumping a particle-laden fluid from a fluid source, thesystem comprising: (a) a pump connected to a source of clean fluid; (b)a double-piston pressure transfer arrangement, said pressure transferarrangement comprising: (i) a first cylinder assembly comprising a firsthollow cylinder, and a first piston in sliding engagement within saidfirst hollow cylinder so as to at least partially define a first chamberand a second chamber; (ii) a second cylinder assembly comprising asecond hollow cylinder, and a second piston in sliding engagement withinsaid second hollow cylinder so as to at least partially define a thirdchamber and a fourth chamber; and (iii) at least one piston rodinterconnecting said first piston and said second piston; and (c) a flowselector associated with said pump, said first chamber of said firstcylinder and said third chamber of said second cylinder, said flowselector assuming alternately: (i) a first selector state in which theclean fluid from said pump is directed to said first chamber so as toact on said first piston, thereby applying pressure to a quantity of theparticle-laden fluid within said second chamber for delivery through anoutlet in a first power stroke, said first power stroke also expelling aquantity of the clean fluid from said third chamber and taking in aquantity of the particle-laden fluid to said fourth chamber; and (ii) asecond selector state in which the clean fluid from said pump isdirected to said third chamber so as to act on said second piston,thereby applying pressure to a quantity of the particle-laden fluidwithin said fourth chamber for delivery through an outlet in a secondpower stroke, said second power stroke also expelling the quantity ofthe clean fluid from said first chamber and taking in a quantity of theparticle-laden fluid to said second chamber, wherein said first andsecond pistons have an inner face facing, respectively, said first andthird chambers and an outer face facing, respectively, said second andfourth chamber, said outer face having larger effective surface areathan said inner face, such that pressure in said particle-laden fluidremains lower than pressure in said clean fluid.